Quantitative information about a material in a region of space may be obtained remotely by observing how the material scatters radiation projected through that region. The scattering process normally causes the light to be reemitted diffusely in all directions though. Therefore, it is often difficult for a detection system to resolve the scattered light arising from one radiation source from the scattered light arising from another source.
One method that can be used to discriminate between scattering sources is to use radiation sources, for example, lasers, with different wavelengths. Then the scattered radiation can be divided into several beams, each beam falling on one of several detectors, each detector responding to only one wavelength. The primary problems associated with using different wavelengths are that multiple detectors and multiple lasers are used. The laser may have distinct properties, including power, size, emitting area, and/or emission patterns, which complicates calibration of the individual detectors, and compromises accuracy.
A technique for measuring particle velocity is Laser Doppler Velocimetry or Anemometry. Two coherent beams are projected at a small angle with respect to each other, and in the small region where they overlap, interference fringes form parallel to their mean direction. When a particle passes through this region, the amplitude of the scattered signal is modulated with a frequency that is equal to the speed divided by the fringe spacing. It is not possible to infer the velocity component (the speed component normal to the fringe pattern including sign) from this information without precisely shifting the frequency of one beam relative to the other. In this case, the fringe pattern drifts toward one direction, and if the drift is faster than the largest speed in that direction, all the measurements of speed relative to the fringe motion will have the same sign. Adding the fringe velocity to the measured relative velocity produces the normal velocity including the correct sign.
While it is possible to use these instruments in situ with naturally occurring particles, it is not convenient. For example, since one requires coherence of the two beams, a slight change in the optical path associated with window degradation or a discontinuity in the medium can distort the fringe pattern. Furthermore, the naturally available particle distribution may not be suitable, for example, too many particles or the wrong size particle. Finally, since coherent beams require high quality lasers, the devices are relatively expensive.